(1) Field of the Invention
The present invention relates to an image forming apparatus, in particular relating to an image forming apparatus in which electro-photographically forms printouts by visualizing image information with a developer and fixing the image information on recording medium by fixing rollers.
(2) Description of the Prior Art
Conventionally, an image forming apparatus such as a copier, printer or the like achieves an image forming process by the steps of: developing an electrostatic latent image of an original image written on a photoreceptor drum, with a developer supplied from a developing unit; transferring the developer image (toner image) onto a recording medium such as recording paper fed from a paper feed cassette; fusing and fixing the toner image transferred on the recording paper by means of a fixing unit made up of a heat roller and a pressing roller, for example, and finally outputting the printed paper.
Generally, the fixing unit used for an image forming apparatus includes, as its essential components, a heat roller, a pressing roller and a heat source for heating the heat roller. With the thus configured fixing unit, a recording sheet with a toner image of unfixed toner formed thereon is led to the contact nip between the heat roller and pressing roller, so that the toner image on the recording paper receives heat from the heat roller surface as it is being passed through the contact nip and the developer is fused and fixed to the recording paper by the pressing force from the paired rollers.
In recent years, in the field of image forming apparatuses, high-speed type machines capable of printing out 80 sheets per minute or greater have been under development. The fixing unit for fixing toner images to recording paper in an image forming apparatus absolutely needs to fuse and fix toner images (image information) onto the recording paper being conveyed, however there is a limit to the condition of installation under which the image forming apparatus is installed. That is, in general, since the specification of the usual power supply is 100 V, 15 A, the maximum power consumption is limited under 1500 W, in the existing circumstance. Accordingly, it is impossible in the status quo to expect drastic enhancement of the power to support high-speed configurations.
To deal with this situation, in order to efficiently use the power for fixing, there has been a generally known configuration in which a main heater (the heater for small-sized paper) and a sub heat (the heater used in combination with the main heater to deal with large-sized paper) are provided inside the heat roller so as to efficiently heat the heat roller.
Also, there is a known conventional configuration in which a heat roller is formed of a thick-wall roller capable of accumulating a predetermined amount of heat or a thin-wall roller capable of quickly transferring heat from a heat source to the roller surface while a pressing roller is formed of a metal core and an elastic material (typically, made of rubber) covered thereon to enhance the sticking effect of fused toner to the recording paper being conveyed, to thereby apply a predetermined pressure onto the heat roller (Patent document 1: Japanese Patent Application Laid-open 2004-144224).
Now, one configurational example of a conventional fixing portion will be described with reference to the drawings.
FIG. 1 is a schematic side view showing a configuration of a heat roller and a pressing roller which constitutes a fixing unit mounted in a conventional image forming apparatus, and FIG. 2 is a schematic sectional view showing the structure of the heat roller and pressing roller.
A fixing unit 130 mounted in an image forming apparatus includes a heat roller 131 and a pressing roller 132, as shown in FIGS. 1 and 2, and is adapted to fuse and fix a toner image that has been transferred on paper P in an unillustrated image forming portion, by rotating heat roller 131 with the recording paper (to be referred to as paper, hereinbelow) P held between heat roller 131 and pressing roller 132 so as to allow the paper to pass between heat roller 131 and pressing roller 132.
In the above way, heat roller 131 and pressing roller 132 which constitute the fixing rollers are arranged so as to hold paper P therebetween with a predetermined pressure when the paper P passes through the nip.
Arranged on the outer periphery of heat roller 131 are a non-contact type thermistor 135a for detecting the surface temperature around the center of the outer peripheral surface and a contact type thermistor 135b for detecting the surface temperature near the roller end.
Heat roller 131 is composed of a hollow cylindrical metal core 131a and an elastic part 131b of a material having heat resistance such as silicone rubber which covers the outer peripheral side of metal core 131a with a predetermined thickness. A main heater 133 and a sub-heater 134 are arranged in the interior space of metal core 131a in the extended direction of heat roller 131's axis (there will be some cases in which this direction is referred to as “axial direction”, hereinbelow).
Main heater 133 has a heater element 133a around the center, designated at 131c, in the axial direction of heat roller 131 so as to heat the center 131c and thereabout of heat roller 131.
On the other hand, sub heater 134 includes a pair of heater elements 134b and 134c extended from respective ends 133b and 133c of heater element 133a of main heater 133 to both ends 131d and 131e along the axial direction of heat roller 131 so as to heat both ends 131d and 131e of heat roller 131 where the heat roller 131 opposes the side edge portions of large-sized paper.
Main heater 133 is mainly used to fix small-sized paper, and sub heater 134 is used in combination with main heater 133 to fix large sized paper.
The thus constructed main heater 133 and sub heater 134 are controlled by an unillustrated heater temperature controller so as to keep the surface temperature of heat roller 131 at the predetermined temperature in conformity with the paper size when the developer image is fixed to the paper.
Pressing roller 132 is composed of a solid cylindrical metal core 132a and an elastic part 132b of a material having heat resistance such as silicone rubber which covers the outer peripheral side of metal core 132a, forming a cylindrical structure having an essentially uniform outside diameter. The pressing roller 132 is arranged in parallel with, along the axial direction of, and abutted against, heat roller 131.
Here, one example of temperature control of heat roller 131 will be shown.
FIG. 3 is an illustrative view showing the temperature distributions on the surfaces of a heat roller and a pressing roller in an image forming apparatus in its operation ready mode; FIG. 4 is an illustrative view showing heat supply distributions of heaters of the heat roller; FIG. 5 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and the pressing roller when small-sized paper is fixed by the fixing unit; and FIG. 6 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and the pressing roller when large-sized paper is fixed by the fixing unit.
When the image forming apparatus is ready for operation, main heater 133 and sub heater 134 are controlled so that the surface temperature of heat roller 131 is maintained at approximately 200 deg. C. uniformly along the axial direction, as shown in FIG. 3 while the surface temperature of pressing roller 132 is kept at approximately 120 to 130 deg. C. along the axial direction.
As shown in FIG. 4, sub heater 134 is adapted to have such a heat supply distribution that more heat is generated at both ends of heat roller 131 (areas A2 indicated by the broken line in the drawing), taking into account heat radiation from the roller ends.
Usually, the surface temperature of the heat roller in the image forming apparatus is controlled so as to be set at approximately 180 to 200 deg. C. (designated fixing temperature) as the necessary temperature set for fixing, whereas the surface temperature of the pressing roller is not controlled, and generally falls within the range of approximately 120 to 150 deg. C., as a result of heat transfer via the press contact (fixing nip) between the heat roller and pressing roller.
In the above configuration, when a print request is made to the image forming apparatus, the surface temperatures of heat roller 131 and pressing roller 132 lower in the area where the paper passes through as shown in FIGS. 5 and 6, because heat is taken from the rollers by the passage of paper and by being used for fusing and fixing unfixed toner.
When the surface temperature of heat roller 131 has become lower than the designated fixing temperature, temperature control is performed so as to effect a stable fixing process. As shown in FIGS. 5 and 6, the part where the temperature lowers corresponds to the area, on each roller, which the paper passing through the nip comes into contact with (passage paper contact area).
However, since no reduction in temperature takes place in the passage paper non-contact areas of each roller, if a temperature raising control is performed in order to compensate for the temperature reduction in the passage paper contact part, the temperatures of heat roller 131 and pressing roller 132 are raised excessively, particularly, in the passage paper non-contact areas, causing the problem that the temperature at both ends of the rollers becomes higher than the designated fixing temperature.
When continuous feed of paper is done with the fixing unit of this configuration, the temperature of the heat roller is raised in the passage paper non-contact areas, resultantly causing degradation of heat roller 131 and pressing roller 132 in their life characteristics.